Process for forming a metal oxide semiconductor devices

ABSTRACT

A method for forming a semiconductor device such as a MOSFET. The method includes forming gate electrode pillars on a silicon substrate via material deposition and etching. Following the etching step to define the pillars, an epitaxial silicon film is grown on the substrate between the pillars prior to forming recesses in the substrate for the source/drain regions of the transistor. The epitaxial silicon film compensates for substrate material that may be lost during formation of the gate electrode pillars, thereby producing source/drain recesses having a configuration amenable to be filled uniformly with silicon for later forming the source/drain regions in the substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of co-pending U.S. patent applicationSer. No. 13/181,575, filed on Jul. 13, 2011, the contents of which ishereby incorporated by reference as if set forth in its entirety.

FIELD OF THE INVENTION

The present invention generally relates to semiconductors, and moreparticularly to a method for fabricating a semiconductor device such asa MOSFET.

BACKGROUND

Gated semiconductor devices such as metal oxide semiconductorfield-effect transistors (MOSFETs) are commonly formed in activeisolated regions of an IC chip. In MOSFETs, dopant implanted source anddrain regions (S/D) are formed in a silicon substrate with correspondingS/D terminals. MOSFETs further include raised or elevated conductivegate layer pillars on the silicon substrate for forming gate electrodes.The gate layer is comprised of conductive material such as doped orundoped polysilicon and is electrically isolated from the siliconsubstrate by a suitable dielectric gate insulator or oxide layer such assilicon dioxide. When a sufficiently high gate voltage is applied, anelectrically conductive inversion layer or channel forms at theinterface between the gate oxide layer and silicon substrate. Theconductive channel extends between the source and the drain, wherebycurrent flows through the channel when a voltage is applied between thesource and drain.

The source and drain regions may be formed in the silicon substrate bydopant ion implantation with P-type or N-type impurities as is wellknown in the art to form NMOS or PMOS transistors, respectively.

These foregoing MOSFET devices, including the raised gate pillars, aregenerally produced using a combination of known photolithography,material deposition, and material removal (e.g. etching, ashing, wetstripping, etc.) fabrication steps. Fabrication processes for formingsemiconductor devices such as MOSFETs may also generally include growingor forming one or more pure monocrystalline epitaxial silicon layers onthe silicon substrate as is well known in the art. Epitaxial siliconlayers beneficially forms uniform silicon layers of predictableelectrical characteristics, thereby enhancing the integrity andperformance of the resulting semiconductor device formed on thesubstrate.

As the size of IC chips continues to decrease, so does the size ofsemiconductor devices such as MOSFETs. This creates new chip fabricationchallenges for producing MOSFETs of ever decreasing dimensions thatremain free of defects which may degrade device performance andtherefore increase chip rejection rates. The challenge of producingdefect-free semiconductor devices is increasingly acute in the N20technology node (20 nm process) and beyond.

One problem facing N20 node fabrication of MOSFET devices is siliconloss. FIG. 1 illustrates a conventional MOSFET device fabricationformation process used to make a lightly doped drain (LDD) MOSFETtransistor which may experience a silicon loss defect. In LDDstructures, source and drain regions near the conductive channel areless heavily doped (e.g. N—) than regions of the source and drainfarther away from the channel. The lower doping near the gate electrodeminimizes electric field effects near the drain, thereby improving thespeed and reliability of the MOSFET transistor.

As shown in FIG. 1, the steps may sequentially include polysilicon layeretching to form gate electrode pillars, LDD ion implantation (shown indashed lines) and subsequent photoresist (PR) stripping, offset sidewall(SW) spacer deposition and subsequent etching, source/drain (S/D) recessetching, and S/D epitaxial silicon deposition. During the variousforegoing material etching and stripping operations, a reduction in thesilicon substrate and gate oxide material occurs as a side effect asshown by the shallow concave depressions between the raised gatepillars). This may result in a profile wherein concave cavities orrecesses are formed beneath the raised source and drain pillars betweenthe gate oxide and substrate (see S/D Recess Etch step). During thesubsequent epitaxial silicon deposition process shown in the last imageof FIG. 1 (labeled S/D Epi), the silicon may not always completely fillthe concave recess resulting in an unwanted void at the very insidecorner of the concave recess as shown in the enlarged detail. Thesevoids are an undesirable defect because if large enough, they mayadversely affect the performance of the MOS transistor.

An improved process for producing a semiconductor device is thereforedesired.

SUMMARY

An improved fabrication process is provided for forming a semiconductordevice such as a MOS transistor which is free of void defects at thegate electrode. The preferred process forms an epitaxial silicon layerprior to etching the source and drain region recesses in the siliconsubstrate. This reconstructs a planar or flat top surface on thesubstrate that compensates for any material loss occurring from thesubstrate as a byproduct of preceding etching and stripping processsteps used to form the gate electrode pillars.

According to one embodiment, a method for fabricating a semiconductortransistor comprising the steps of: providing a silicon substrate;forming a gate oxide layer on the semiconductor substrate; forming aconductive gate layer on the gate oxide layer; forming a patternedphotoresist layer above the gate layer, the patterned photoresist layerhaving a gate electrode pattern; performing a first etching step to forma plurality of recesses in the gate layer defining raised gate electrodepillars corresponding to the gate electrode pattern; implantingimpurities through the recesses into the substrate to form lightly dopeddrain regions; forming an epitaxial silicon film on the substrate; andperforming a second etching step after forming the epitaxial siliconfilm to form source and drain recesses in the substrate. The epitaxialsilicon film may be removed during the second etching step.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the preferred embodiments will be described withreference to the following drawings where like elements are labeledsimilarly, and in which:

FIG. 1 shows sequential steps in a conventional prior art semiconductordevice fabrication process;

FIGS. 2-8 show sequential steps in an exemplary semiconductor devicefabrication process according to the present invention;

FIG. 9 is a detailed view of a gate electrode formed by the prior artprocess of FIG. 1;

FIG. 10 is a detailed view of a gate electrode formed by the process ofFIGS. 2-8;

FIG. 11 shows an optional embodiment of a semiconductor device with anoxide sidewall liner; and

FIG. 12 shows a finished semiconductor device such as MOSFET produced bythe process described herein.

All drawings are schematic and are not drawn to scale.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This description of illustrative embodiments is intended to be read inconnection with the accompanying drawings, which are to be consideredpart of the entire written description. In the description ofembodiments disclosed herein, any reference to direction or orientationis merely intended for convenience of description and is not intended inany way to limit the scope of the present invention. Relative terms suchas “lower,” “upper,” “horizontal,” “vertical,”, “above,” “below,” “up,”“down,” “top” and “bottom” as well as derivative thereof (e.g.,“horizontally,” “downwardly,” “upwardly,” etc.) should be construed torefer to the orientation as then described or as shown in the drawingunder discussion. These relative terms are for convenience ofdescription only and do not require that the apparatus be constructed oroperated in a particular orientation. Terms such as “attached,”“affixed,” “connected” and “interconnected,” refer to a relationshipwherein structures are secured or attached to one another eitherdirectly or indirectly through intervening structures, as well as bothmovable or rigid attachments or relationships, unless expresslydescribed otherwise. Moreover, the features and benefits of theinvention are illustrated by reference to the preferred embodiments.Accordingly, the invention expressly should not be limited to suchpreferred embodiments illustrating some possible non-limitingcombination of features that may exist alone or in other combinations offeatures; the scope of the invention being defined by the claimsappended hereto.

One embodiment of an exemplary process for forming a semiconductordevice such as without limitation a lightly doped drain (LDD) MOSFET isillustrated in FIGS. 2-8. These figures show sequential cross-sectionalside views through the semiconductor structure during the MOSFET deviceformation process. In this non-limiting example, process steps occurringduring the formation of a LLD MOSFET device are shown and described.However, it will be appreciated that the present process may be used toproduce numerous other type transistors and semiconductor devices inwhich raised gate electrode pillars or other pillar structures may beformed. The semiconductor transistors to which the present process isapplicable includes without limitation NMOS, PMOS, CMOS, and BiCMOSdevices.

In one embodiment, the present process may include one or more epitaxialmaterial layer formation steps. Advantageously, embodiments of thefabrication process according to the present invention produce asemiconductor device that is preferably free or essentially free of voiddefects as described in the Background of the Invention. This improvesthe integrity of the semiconductor device and reliability.

The epitaxial silicon layer formation processes (“epi processes”)described herein can be performed in any suitable commercially-availableepitaxy tool, such as the Epsilon Model Epitaxy Tool available from ASMInternational of the Netherlands, or other suitablecommercially-available semiconductor fabrication tools.

Beginning now with initial reference to FIG. 2, a multi-layersemiconductor device 10 may initially be created by sequentially forming(from the bottom upwards) an insulating dielectric gate oxide layer 30on a conventional semiconductor silicon substrate 20, a conductive gatelayer 40 thereon, and a hard mask layer 50 thereon. A patternedphotoresist layer 60 is formed directly on the hard mask 50, which maybe patterned using conventional photolithography techniques as wellknown to those skilled in the art. This may include in some embodimentsusing a mask or reticle supported above the photoresist layer, a lightsource to expose selective portions of the photoresist material, andconventional process steps for removing the developed portions of thephotoresist as are well known to those in the art. The patternedphotoresist layer 60 will be used to ultimately form the gate pillars onsubstrate 20 for the gate electrodes of a MOSFET transistor, assubsequently described herein. Accordingly, the photoresist pattern isconfigured and shaped to provide a gate electrode pattern.

Substrate 20 may be any suitable material conventionally used forfabricating MOSFET transistors. In one exemplary embodiment, a dopedsilicon substrate 20 may be provided that may be doped with suitableP-type or N-type impurities depending on whether an NMOS or PMOS devicerespectively is to be formed.

Gate oxide layer 30 may be any suitable thin film of insulatingdielectric material commonly used in semiconductor fabrication processesfor insulating layers. In one embodiment, the gate oxide layer 30 may besilicon dioxide which may be formed by thermally oxidizing the siliconin the substrate to form a thin insulating layer of silicon dioxide thatis able to electrically isolate the transistor gate layer 40 from thedrain and source regions in substrate 20. In representative examples,gate oxide layer 30 may have a typical thickness of about 10-100 A.

Conductive gate layer 40 may be any suitable conventional material usedin semiconductor fabrication processes for transistor gates, such aswithout limitation a doped or undoped polysilicon layer,metal-containing layer, combinations thereof, or other conductive layerthat is able to serve as a transistor gate for controlling the flow ofelectrons between the source and drain of the semiconductor device. In apreferred embodiment, gate layer 40 may be polysilicon. Gate layer 40may be doped or undoped. If doped, a N/PMOS device may have a gate layer40 doped with phosphorus (n-type dopant) or boron (p-type dopant). Inrepresentative examples, transistor gate layer 40 may have a thicknessof about 300 A-2000 A.

In other possible embodiments, gate layer 40 may be a metal such asaluminum, highly doped silicon, refractory metals such as tungsten, asilicide (e.g. TiSi, MoSi, TaSi or WSi) or multiple layers of theforegoing materials.

Hard mask layer 50 may be any suitable conventional material used insemiconductor fabrication processes for hard marks, such as for examplewithout limitation silicon nitride (SiN), (silicon carbide) SiC, andsilicon oxynitride (SiON). Hard mask layer 50 protects the polysiliconlayer during various etching and material removal steps. The hard mask50 when patterned also provides a gate hard mask pattern for forming thegate electrode pillars as further described herein. In representativeexamples, hard mask layer 50 may have a thickness of about 50 A-1500 A

Gate oxide layer 30, gate layer 40, hard mask layer 50 and photoresistlayer 60 may be deposited in the formation of semiconductor 10 by anysuitable conventional method as well known to those skilled in the artof semiconductor fabrication, such as for example without limitationPVD, PECVD, CVD, MVC, ECP, thermal oxidation processes, or others. It iswell within the ambit of those skilled in the art to select anappropriate method for forming the foregoing material layers.

Following formation of the basic semiconductor structure resulting andshown in FIG. 2, the process steps involving the addition of epitaxialsilicon films and layers for creating a semiconductor device such as aMOSFET without void defects will now be described.

After providing the semiconductor structure 10 shown in FIG. 2, apreferably anisotropic etching step is next performed as shown in FIG. 3using a suitable etching gas (denoted “EG” in the figures) process totransfer the gate electrode pattern from photoresist layer 60 into thehard mask layer 50 and underlying transistor gate layer 40 resulting inthe semiconductor structure shown in FIG. 3. This etching step producesrecesses 41 in the semiconductor 10 which define the gate pillars 43 forcreating the gate electrodes. Photoresist 60 is preferably moreresistant to etching by the etching gas chemistry used and thereforeprotects the portions of gate layer 40 and hard mask 50 below whichbecome the gate pillars 43. As shown, the anisotropic etching stepproduces and defines essentially vertical sidewalls 42 on the pillars43, a portion of which will subsequently be covered by a sidewall spacerin subsequent process steps as further described herein.

In one possible embodiment, anisotropic dry gas plasma etching may beused. Dry etching is performed in an etcher machine by applying anelectromagnetic energy source (such as RF) to a gas containing asuitable chemically reactive element that reacts with the material to beetched or removed. The gas plasma emits positively charged ions thatstrike and dissolve the dielectric material. By using a combination ofhard masks and/or patterned photoresist layers above the conductive gatelayer 40, various patterns of recesses or openings in the gate layer 40or silicon substrate 20 can be made since the conductive gate materialbeneath the hard mask and photoresist will not substantially dissolve.Because the ions strike the gate material essentially perpendicular toits surface in anisotropic dry plasma etching, vertical recess profilescan be created with virtually no undercutting beneath the hard mask andphotoresist

Preferably, by relying on material etching selectivity, materialsselected for both hard mask layer 50 and transistor gate layer 40 arepreferably susceptible to etching by conventional anisotropic dry gasplasma etching chemistries, such as without limitation elementalfluorine F, CF₄, C₄F₈, CHF₃, CH₂F₂, or C₅F₈. Other suitable etching gaschemistries, however, may also be used. The type of etching gas usedwill depend on the material selected for both hard mask layer 50 andunderlying gate layer 40, both of which preferably are susceptible toetching using the same etching gas so that a single etching chamber maybe used. Preferably, photoresist 60 is resistant to the etching gas EGalthough some material loss may occur, but at a greatly reduced ratecompared to hard mask 50 and gate layer 40.

The first etch step shown in FIG. 3 is preferably controlled to stopetching near the top interface surface of gate oxide layer 30 orslightly into that layer. In one possible embodiment, this may beachieved by selecting an appropriate operating mode of thecommercially-available etching tool that may be used in the process. Theetching tool may be set to “end point detected mode.” When the etchingtool detects a different signal being received from gate oxide layer 30(shown in FIG. 3) in the etching process, the system will be stopped toprevent over-etching.

Although the etching process in FIG. 3 is preferably carefullycontrolled and stopped when nearing the top of now exposed interfacesurface of gate layer 30, some over-etching may nevertheless occurresulting in undesired material loss from gate oxide layer 30 andunderlying silicon substrate 20 that may form concave depressions 45 inrecesses 41 between gate electrode pillars 43.

In the next step, referring to FIG. 4, in one embodiment LDD regions 48(shown dashed) may be formed in silicon substrate 20 via implantingimpurities using conventional ion implantation with a suitable dopantcontaining, for example, N-type or P-type impurities as will be wellknown to those skilled in the art. P-type regions may be formed by theuse of P-type dopants such as boron (B) or other elements of group II orIII of the periodic table. N-type regions may be formed by use of N-typedopants such as phosphorus (P), arsenic (As) or other elements of groupV or VI of the periodic table.

With continuing reference to FIG. 4, photoresist layer 60 is thenstripped after forming LLD regions 48 by any suitable technique commonlyused in the art including gaseous or wet etching methods. In someembodiments, without limitation, photoresist layer 60 may be strippedusing a dry gas plasma containing for example H2, N2, O2, or CF4.Alternatively a wet photoresist stripping process may be used, such aswithout limitation SPM (high density and high temperature Sulfuric acid(H2SO4) and hydrogen peroxide (H2O2) solution). The photoresiststripping does not substantially effect hard mask layer 50 andunderlying transistor gate layer 40 which preferably are both made ofmaterials selected to be resistant to material loss/reduction by thephotoresist stripping process selected. However, the oxidant in theforegoing stripping processes will further oxidize substrate 20 siliconand cause further silicon loss. The photoresist stripping processes,therefore, may result in yet a second instance of further silicon lossand deepening of concave depressions 45 from exposed gate oxide layer 30and silicon substrate 20 between gate electrode pillars 43.

Following the LDD ion implantation and photoresist stripping processes,the semiconductor structure generally appears as shown in FIG. 4.

It will be appreciated that in some alternative processes, hard masklayer 50 may first be patterned using patterned photoresist 60 as shownin FIG. 2, then prior to the etching step shown in FIG. 3, thephotoresist may be stripped and then the patterned hard mask having thesame configuration as in FIG. 3 may then alone be used to etch the gatelayer 40 to form gate pillars 43. Either process flow described hereinare typically used in the production of MOSFETs and may be followed.

Following the patterned photoresist layer 60 stripping process, exposedportions of gate oxide layer 30 are next removed from between gatepillars 43 leaving only gate oxide remaining beneath transistor gatelayer 40 in gate pillars 43. The semiconductor MOSFET structure wouldappear as in FIG. 4 with concave depressions 45 in recesses 41 betweengate electrode pillars 43, but with the gate oxide layer 30 absent frombetween the gate pillars. This will pre-clean and prepare the siliconsubstrate 20 for a subsequent epitaxial silicon film formation step torebuild the substrate 20 between the gate pillars 43 and eliminateconcave depressions 45.

Any suitable method wet or dry cleaning methods commonly used in the artfor substrate pre-cleaning and removing a gate oxide material may beused for removing exposed gate oxide layer 30 between gate pillars 43and prepare the surface of silicon substrate 20 for forming an epitaxialsilicon layer. In some exemplary embodiments, a conventional wet DHF(dilute hydrofluoric acid) etching or cleaning etch process may be used.In other embodiments, a dry etching or cleaning process may be used suchas without limitation a plasma-free gas chemical etch system targeted atselective oxide film etching using a Certas machine available from TokyoElectron Limited (TEL) of Japan, or a SiCoNi etching process using amachine from Applied Materials of Santa Clara, Calif. using HF+NH3vapor/HF+NH3 plasma to remove the gate oxide.

Preferably prior to deposition of the offset sidewall spacer materialand subsequent etching step used to configure and form the verticalsidewall spacers 44 as shown in FIG. 6, a first selective epitaxialsilicon film formation step is next performed in a preferred embodimentto rebuild the reduced silicon substrate 20 and gate dielectric layer30. This step will fill concave depressions 45 to re-establish theoriginal thickness and flat top surface profile of substrate 20 byreplenishing material lost in the prior etching and photoresiststripping in FIGS. 3 and 4. Preferably, the epitaxial silicon is grownuntil the concave depressions 43 between the gate electrode pillars 43are eliminated, thereby forming a generally flat horizontal surface ofan epitaxial silicon film 70 on substrate 20 as shown in FIG. 5. This isimportant in preventing or minimizing the formation of void defectsbeneath the source and drains pillars, as further explained herein.

FIG. 5 shows the semiconductor structure 10 created by the firstepitaxial silicon film formation or growth. In one embodiment, epitaxialmaterial may be grown by silicon selective epitaxial growth (SEG), whichis well known in the art. This silicon SEG process grows a thin siliconfilm 70 on the silicon substrate 20 which fills in the concavedepressions formed between gate pillars 43 that are caused by the prioretching and stripping steps (see, e.g. FIGS. 3 and 4), which may causethe void defect described herein. Preferably, the SEG process forms asubstantially flat top surface on silicon film 70 in the bottom ofrecesses 41 between the gate electrode pillars 43 as shown in FIG. 5.The silicon SEG process generally uses a reactant gas comprising asilicon precursor source material, an inert carrier gas, and optionallya dopant. The silicon SEG process used preferably suppresses theformation of silicon on gate layer 40 which may be made of polysilicon,yet selectively forms silicon film 70 on substrate 20 as shown.

In one exemplary embodiments, the SEG process may use a DCS(dichlorosilane) precursor, HCl etching gas, and H2 carrying gas. Theselectivity is achieved by HCl which etch amorphous Si faster thancrystal silicon. Exemplary process pressures of 1 to 100 torr attemperatures of 500 C to 900 C may be used in some embodiments.

Referring now to FIG. 6, semiconductor structure 10 is shown havingoffset vertical sidewall spacers 44 formed on the gate pillars 43 andadjacent hard mark 50. Offset vertical sidewall spacers 44 are used tocontrol and define S/D boundary next to gate in order to obtain maximumdevice performance advantage without production issues. The process offorming sidewall spacers 44 is well known in the art. Beginning with thesemiconductor structure shown in FIG. 5, a layer of sidewall spacermaterial is deposited over the entire structure in FIG. 5 by anysuitable method commonly used in the art which preferably completelyfill the recesses 41 and covers the top of the gate pillars 43 and hardmask layer 50 thereon. Suitable methods commonly used in the art fordepositing the sidewall spacer material includes without limitationchemical vapor deposition (CVD), physical vapor deposition (PVD), andPlasma-Enhanced Chemical Vapor Deposition (PECVD). Other suitablemethods may be used.

The sidewall spacer 44 material layer is thereafter etched using asuitable conventional etching process. In one embodiment, sidewallspacer material layer may be etched using anisotropic dry plasma gasetching having appropriate etching gas chemistry as will be well knownto those in the art, examples of which are already described hereinincluding fluorinated gases. In some embodiments, the sidewall spacermaterial may be, for example without limitation silicon nitride (SiN),(silicon carbide) SiC, silicon oxynitride (SiON), SiCN, BN, SiO2, andSiOCN. The gas chemistry used to etch the sidewall spacer material willbe dependent on the material selected for the sidewall spacers. Theanisotropic etching steps removes the sidewall spacer material in therecesses 41 between gate pillars 43 and on top of the hard mask layer50, thereby leaving only material on the vertical sidewalls 42 of thepillars 43 which create the offset sidewall spacers 44 as shown in FIG.6.

It should be noted that in some embodiments of the MOSFET fabricationprocess, an oxide layer beneath sidewall spacer 44, which is nativeoxide, may form on sidewalls 42 of gate pillar 43 when depositingsidewall spacer material that may not be later removed in subsequentprocessing. A portion of this native oxide may form beneath the verticalsidewall spacers 44 when the spacers are formed as is visible in FIG.10. In some embodiments, an oxide liner 80 may be intentionally coatedon sidewalls 42 before sidewall spacer 44 deposition. If this is thecase, the oxide liner may appear as shown in FIG. 11 in gate sidewall42. The offset sidewall spacers 44 may then be formed as shown in FIG. 6and processing will continue as described herein.

Referring now to FIG. 7, an etching step is next performed to formsource and drain recesses 75 between gate pillars 43 for creating highlydoped regions within substrate 20. In one preferred embodiment,source/drain recesses 75 may be formed by a dry etch first to form arecess depth and then followed by a wet etch to form the desiredprofile. In one exemplary embodiment, dry etching may be performed byusing plasma chemistry gas like HBr/Cl2/O2/N2/NF3/SF6/CxFyHz. Wet etchmay be performed by using an etchant such as TMAH (Tetramethylammoniumhydroxide) with density range from 0.5% to 10% in the temperature of 10C to 100 C.) Other suitable dry and/or wet etching processes commonlyused and well known in the art may be used for forming recesses 75.

With respect to FIGS. 7-8 and 10, it should be noted that the underside74 of the gate oxide layer 30 proximate to lateral sides of the gateelectrode pillars is exposed within the source and drain recesses duringetching of those recesses. The underside 74 of the gate oxide layer 30below the gate layer 40 and vertical sidewall spacers 44, andcorrespondingly the underside of spacers 44 abutting gate oxide layer30, has a generally flat surface profile with generally square lateraledges and no cavities or beveled recessed corners formed beneath thegate electrode pillars 43. This flat profile is more easily filled inthe step in FIG. 8 with epitaxial silicon without formation of theunwanted void defect.

By contrast, the S/D recess etch step in the prior art process shown inFIG. 1 results in downward sloping lateral edges on the oxide layer withbeveled cavities being formed at the inside corner in recesses 75beneath the gate pillars that causes the void defects when subsequentlyfilled with epitaxial silicon (see, e.g. FIG. 9). The underside 74 ofgate oxide layer 30 slope downward proximate to gate electrode pillars43 which forms an acutely angled compound inside corner shape that isdifficult to fill with epitaxial silicon as seen in FIG. 9 leaving thevoid defect.

After formation of source and drain recesses 75, a second epitaxialsilicon formation step is performed to preferably completely fillrecesses 75 with silicon to form doped source and drain regions 76 asshown in FIG. 8. Any suitable conventional epitaxial silicon processknown in the art for forming source/drain regions may be used. Theprocesses preferably need not preferably be selective Si epitaxy so thatan overfill of recesses 75 intentionally results in some embodiments.The overfill of gate and drain regions 76 is intentional for contactformation and better current crowding. The overfill is formed bybottom-up gap filling. The doped source and drain regions 76 may beformed in the epitaxial silicon drain regions 76 by dopant ionimplantation with P-type or N-type impurities as is well known in theart to form NMOS or PMOS transistors, respectively. Contacts 100 (see,e.g. FIG. 12) may later be formed on doped source/drain regions 76 in aconventional manner.

FIG. 8 and enlarged detail shown in FIG. 10 show semiconductor structure10 according to the new process. The void defect beneath the lateraledges of the gate pillars 43 and gate oxide 30 are avoided due to theimproved flat profile beneath the pillars created by the first selectiveepitaxial silicon film 70 formation step shown in FIG. 5 and describedabove. This corrects the profile of the substrate 20 to compensate forthe silicon loss caused by the first gate layer (e.g. polysilicon)etching step and photoresist stripping described herein.

By contrast, FIG. 9 shows an enlarged detail of the semiconductorstructure formed using the prior art process having the void defect.

Subsequent conventional MOSFET processing steps may subsequently beperformed to complete the transistor, including deposition of apre-metal dielectric (PMD) oxide layer 90 and formation of thedrain/source metal contacts 100 formed on source/drain regions 76 toproduce a finished device as shown in FIG. 12. Completely formed polysilicon or metal gates 110 are further shown with hard mask layer 50removed by any suitable conventional means used in the art.

EXAMPLES

Several MOSFET fabrication test process were conducted to compare thefollowing three scenarios: (1) the prior art fabrication process shownin FIG. 1 which experiences silicon loss or void defect formationwithout benefit of any intermediate selective epitaxial siliconformation step to rebuild the silicon substrate as shown in FIG. 5according to the present invention; (2) the fabrication processaccording to the present invention with formation of a 1 nm epitaxialsilicon layer 70; and (3) the fabrication process according to thepresent invention with formation of a 3 nm epitaxial silicon layer 70.The silicon losses were estimated in each of the three test scenarios.

In the prior art process test scenario (1), an estimated silicon loss of10-12 Angstroms resulted below the gate electrode pillar. In processtest scenario (2) according to the present invention, the estimatedsilicon loss was estimated to be about 0 Angstroms. In process testscenario (3) according to the present invention, an estimated excess ofsilicon of about 10 Angstroms resulted.

Accordingly, in one embodiment a preferred exemplary non-limitingthickness of an epitaxial silicon film 70 is at least about 1 nm whichwas effective to fill concave recesses 43 between the gate pillars 43and eliminate the void defect. Silicon films 70 greater than 1 nm may beused as the test scenario (3) demonstrated that the void defect wasavoided albeit an excess of silicon material was formed beneath the gateelectrode pillar.

While the foregoing description and drawings represent preferred orexemplary embodiments of the present invention, it will be understoodthat various additions, modifications and substitutions may be madetherein without departing from the spirit and scope and range ofequivalents of the accompanying claims. In particular, it will be clearto those skilled in the art that the present invention may be embodiedin other forms, structures, arrangements, proportions, sizes, and withother elements, materials, and components, without departing from thespirit or essential characteristics thereof. One skilled in the art willfurther appreciate that the invention may be used with manymodifications of structure, arrangement, proportions, sizes, materials,and components and otherwise, used in the practice of the invention,which are particularly adapted to specific environments and operativerequirements without departing from the principles of the presentinvention. In addition, numerous variations in the preferred orexemplary methods and processes described herein may be made withoutdeparting from the spirit of the invention. The presently disclosedembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingdefined by the appended claims and equivalents thereof, and not limitedto the foregoing description or embodiments. Rather, the appended claimsshould be construed broadly, to include other variants and embodimentsof the invention, which may be made by those skilled in the art withoutdeparting from the scope and range of equivalents of the invention.

What is claimed is:
 1. A method for fabricating a MOS transistorcomprising the steps of: (a) providing a silicon substrate; (b) forminga dielectric gate oxide layer on the semiconductor substrate; (c)forming a conductive gate layer on the gate oxide layer; (d) forming ahard mask layer on the gate layer; (e) forming a patterned photoresistlayer above the gate layer, the patterned photoresist layer having agate electrode pattern; (f) forming recesses in the hard mask and gatelayer defining raised gate electrode pillars corresponding to the gateelectrode pattern; (g) implanting impurities through the recesses intothe substrate to form lightly doped drain regions; (h) forming anepitaxial silicon film on the substrate between the gate electrodepillars; (i) forming sidewall spacers on the gate electrode pillars; (j)forming source and drain recesses in the silicon substrate to definesource and drain regions, wherein the epitaxial silicon film is removed;and (k) depositing epitaxial silicon in the source and drain regions. 2.The method of claim 1, further comprising a step of removing thephotoresist layer after the implanting step but before the forming anepitaxial silicon film on the substrate step.
 3. The method of claim 1,wherein the epitaxial silicon film has a substantially flat top surfaceprior to forming the source and drain recesses.
 4. The method of claim1, wherein the epitaxial silicon film has a thickness of at least about1 nm.
 5. The method of claim 1, wherein the conductive gate layercomprises polysilicon.
 6. The method of claim 1, wherein the recesses inthe hard mask and the gate layer are formed by an anisotropic etchingprocess.
 7. The method of claim 6, wherein the etching process isanisotropic dry gas plasma etching.
 8. The method of claim 1, whereinthe gate oxide layer is removed between the gate electrode pillars priorto forming the epitaxial silicon film on the substrate.
 9. The method ofclaim 1, wherein the step (h) is by a silicon selective epitaxial growth(SEG) process.
 10. The method of claim 9, wherein the SEG process uses adichlorosilane precursor, HCl etching gas, and H₂ carrying gas.
 11. Themethod of claim 1, wherein the step (h) is a PECVD process.
 12. Themethod of claim 1, further comprising a step of removing the photoresistlayer after the implanting step but before the forming an epitaxialsilicon film on the substrate step.
 13. The method of claim 1, whereinthe impurities are implanted via ion implantation using P-type or N-typeimpurities.
 14. The method of claim 1, wherein the steps (b), (c), (d)and (e) are one of a PVD, PECVD, CVD, MVC, ECP, and thermal oxidationprocess.
 15. The method of claim 1, wherein the step (i) is one of aCVD, PVD, and PECVD process.
 16. The method of claim 1, wherein the step(k) is by an SEG process.
 17. The method of claim 16, wherein the SEGprocess uses a dichlorosilane precursor, HCl etching gas, and H₂carrying gas.
 18. The method of claim 1, wherein the gate electrodepillars having sidewalls and further comprising depositing an oxideliner on the sidewalls of the gate electrode pillars before the step(i).